Benefits of summer sun to reduce risk of multiple sclerosis may extend beyond childhood
'You need safe sun exposure, but at the same time don't be afraid to go outside,' professor says
Children and adults who spend a lot of time outside in the summer may be less likely to develop multiple sclerosis years later, a U.S. study suggests.
Sun exposure is thought to lessen the risk of MS, a chronic disease in which a person's immune system targets nerve cells in the brain and spinal cord, leading to damage. It is estimated Canada may have among the highest prevalence of MS in the world.
While the disease is common, little is known about its causes. But for more than 10 years, sun exposure has been thought to be linked to MS risk.
In Wednesday's online issue of the journal Neurology, Tremlett and her team report combing through data on 151 women with MS and 235 others of similar age without the disease who were all participating in the Nurses' Health Study based in Boston. The long-running U.S. study is one of the largest investigations into risk factors such as diet, hormones, and environment for major chronic diseases in women.
"We found that just generally going out in the summer was a beneficial thing and didn't matter so much if you were exposing yourself to direct sunlight. It was just going out in the summer that was associated with a reduced risk," Tremlett said in an interview.
More time spent outside in the summer appeared to be more important than actual time spent in the sun in minimal clothing, she added.
In the study, subjects were divided into three groups based on low, moderate and high exposure to UV-B rays, which depended on the latitude and altitude of where they lived and the average cloud cover for the location.
Women who lived in the sunniest locations during childhood and adolescence had half the risk of MS as other women, the researchers found.
"Our findings suggest that health behaviours through until early adulthood can alter your risk of developing MS," said Tremlett, who holds the Canada Research Chair in neuroepidemiology and multiple sclerosis.
Tremlett said the study infers a beneficial association, but said people should still be mindful that there are also risks that come with spending time in the sun.
"You need safe sun exposure, but at the same time don't be afraid to go outside," Tremlett said, referring to guidelines from the Canadian Cancer Society, which balance the known risk of developing skin cancer with the benefits of spending some time outside.
Women in the study had answered questions about their current and previous sun exposure. For instance, high summer sun exposure was defined as more than 10 hours per week and more than four hours per week in the winter.
Melatonin matters
The study's findings extend beyond the benefit of how our skin produces vitamin D in summer sunlight, Tremlett said.
Sunlight affects how the immune system functions, which is also involved in MS. What's more, when sunlight hits the retina in the eye, it affects melatonin levels, which can also influence the immune system.
"All those things in moderation can be beneficial to health in general and our findings suggest that it may be helpful as well in terms of reducing MS."
The study wasn't a controlled experiment designed to prove whether UV-B directly protects against MS.
Another limitation is that the majority of participants were fair skinned. That could make a difference to health given that people with darker skin tones need to be out in the sun for longer before they produce vitamin D.
The study also did not include men, and it isn't known whether the findings extend to them.
Tremlett acknowledged that recollections about sunlight exposure may not be perfect, but the fact that the trend was observed across different age groups and by place of residence leads the researchers to believe that the findings are robust.
The study was funded by the University of British Columbia, the Canada Research Chair Program and the U.S. National Institutes of Health.
With files from CBC's Amina Zafar